1. Field of the Invention
The invention relates to camless valve actuators, particularly valve actuators for automotive vehicle internal combustion engines.
2. Background Art
Internal combustion engines for automotive vehicles have power cylinders and piston assemblies that define air/fuel combustion chambers. Each chamber has at least one air/fuel intake valve and at least one exhaust valve. In the case of a four-stroke cycle engine, the intake valve is opened during the intake stroke to admit an air/fuel mixture; and it is closed during the compression, power and exhaust strokes of the piston. The exhaust valve is opened during the exhaust stroke of the piston; and it is closed during the compression, power and intake strokes. The intake and exhaust valves are sequentially operated in known fashion to effect the usual Otto cycle as power is transferred from the pistons to the engine crankshaft.
Typically, the intake and exhaust valves are actuated by a camshaft that is connected driveably to the crankshaft with a 2:1 driving ratio.
In the case of a camless valve train, electromagnetic actuators for the intake and exhaust valves have been used for sequentially opening and closing the valves. Electromagnetic actuators for camless valve trains typically have two electromagnets, a closing magnet and an opening magnet, together with an armature situated between opposed pole surfaces. The armature is designed to move between the pole surfaces against forces established by a valve closing spring and a valve opening spring. The spring forces act in opposition, one with respect to the other.
Electromagnetic forces developed on the armature oppose the spring forces. In a non-energized state, the armature is held in equilibrium position between the pole surfaces.
One of the electromagnets has a closing coil, which, when energized, holds the armature against its pole surface. When the closing coil is switched off, the opposing electromagnet, which is an opening coil, is energized, thereby driving the armature to a valve opening position.
When the valve is actuated, the armature and the valve are driven at high velocities as they move toward the opening coil. It is possible, therefore, for the armature to have high impact energy as it engages the opening coil pole face. Similarly, when the closing coil is actuated, the armature may be subjected to high impact energy as the valve is closed. High impact energy results in excessive noise as well as wear on the valves.
If a camless valve train of known designs is calibrated to achieve optimum impact velocities for the purpose of reducing noise and wear, variations in the operating parameters and operating conditions of the engine (including valve wear, temperature changes and hydrocarbon debris buildup) will cause the control of the position and velocity of the armature to deviate from an optimum calibration.
Attempts that have been made to provide more consistent control of electromagnetic valve actuators include the design disclosed in U.S. Pat. No. 6,234,122. Variations in operational system parameters are accounted for in the design of the ""122 patent by sensing a change in the inductance of the electromagnetic coil windings as a measure of impact velocity. A predetermined value of the impact velocity of the armature on the electromagnet is adjusted to a so-called set point by controlling the supply of energy to the electromagnet based on a change in inductance of the electromagnet.
Another attempt to control movement of the armature of an electromagnetic actuator is described in U.S. Pat. No. 6,196,172. That design relies upon a control movement of the actuator armature in accordance with a desired, predetermined trajectory. The acceleration of the armature is calculated as a derivative of the armature velocity. The control of the velocity is achieved in an open-loop fashion determined by operating variables during calibration of the actuator in accordance with the so-called desired trajectory.
In a design described in U.S. Pat. No. 6,003,481, the motion of the armature in the final phase of the armature""s motion is achieved by providing an additional mass that is engaged by the armature when the valve approaches the fully opened position or the fully closed position. The additional mass modifies the opening velocity and the closing velocity of the valve. Movement of the additional mass is modified by a cushioning spring.
The invention comprises a control method for an electromagnetic camless valve train that can be adaptively calibrated for optimal performance. The method of the invention achieves a so-called soft landing of the valve, which avoids the high impact velocities during valve opening and closing. The control method of the present invention reduces impact velocity of the armature as it approaches the catching coil, from about 1 meter per second to 0.1 meter per second for a valve in a contemporary automotive engine. The soft landing velocity relative to the catching coil achieved by the controller is obtained using an electromagnetic PWM signal based upon an optimal proportional control of the position and the instantaneous velocity of the armature, as well as the current in the coil, in a closed-loop, full-state feedback fashion. The controller is characterized by two different stages based upon armature position; i.e., a flux initialization stage and a landing stage. Each stage has its unique function in the control of the optimal overall landing characteristics of the valve and the armature.
In practicing the method of the invention, a position sensor is used to measure the displacement of the armature as the opening and closing coils are alternately activated and deactivated to capture the armature. The valve, which is mechanically coupled to the armature, is biased toward an intermediate position between the coils by at least one spring. Electrical current supplied to each coil is measured as the coil is activated. Current for each coil also can be determined as an observed current that would be a function of coil inductance, voltage and resistance. The instantaneous velocity of the armature is computed as the armature moves toward the catching coil in response to alternating activation of the coils. The activating voltage is computed in a closed-loop fashion as a function of current, displacement and armature velocity whereby the armature approaches the coil pole faces with a controlled movement to achieve reduced impact velocity.